What is Level of Service (LOS) in traffic engineering?

Level of Service is a qualitative measure of traffic operating conditions, rated from A to F. LOS A represents free flow with minimal delays; LOS F represents breakdown with unacceptable delays. LOS is determined differently for various facilities: for signalized intersections based on control delay, for highways based on density and speed, for pedestrian facilities based on pedestrian space. Target LOS depends on area type (urban vs rural) and functional classification, typically LOS C or D for design.